ABSTRACT

Fossil leaves in clayey lake sediments of the Miocene Oviatt Creek Flora, Idaho, were mapped in order to determine their usefulness for indicating paleocurrent flow. Leaves from the Oviatt Creek florule P-42 were measured with respect to their size, orientation, and position within a 0.5-m-thick section over an area of about 20 m² to determine patterns in leaf sorting, orientation, and species distribution. Leaf taphonomy indicates that the P-42 lake deposits represent infilling from a southwestern site by a slow-moving source. Preliminary geological mapping in the area supports this interpretation. None of these leaf parameters could be used alone to make this interpretation. Leaf orientation appears to be reliable only for determining flow lineation. This is because leaf etioles may point either up or down flow. Leaves with heavy petioles (i.e., Platanus) were aligned almost exclusively parallel to flow. Fruits of Acer longer than 4 cm were consistently orientated in the same direction. Both types of plant parts provide useful data to assist in the determination of paleocurrents in fine-grained sediments where primary sedimentary structures are lacking. Also, leaf orientation perpendicular to flow appears to be a possibility in weak currents.